A new report from Australia is being touted as the latest definitive proof that infrasound around wind farms is no louder than infrasound from wind and human activity in areas with no wind farms. While providing a relatively robust new set of data, the study design leaves some important questions raised by wind farm neighbors and other acousticians unanswered.

This may be the most comprehensive infrasound/low-frequency study released yet: it includes several days of measurements made prior to construction of the wind farm, along with at least ten days of measurements made when the wind farm was partly operational, and ten more days once the wind farm was fully operational. Sound was measured down to 0.8Hz, lower than some similar studies. Most importantly, sound was recorded inside the homes, which were1.8km (1.1mi) and 2.7km (2.7mi) from the nearest turbines, on opposite sides of the wind farm; at both homes, there were many more turbines at slightly greater distances than the closest ones.

At the more distant home, daytime infrasound levels prior to construction were commonly 60-70dBG, with a few peaks of 80-90dBG (grey circles below); these measurements capture the natural ambient infrasound levels caused by the wind itself, along with contributions from machinery and vehicles in the area (the threshold for human perception is about 95dBG for pure tones, perhaps lower for pulsing sounds). The peaks were much lower at night than during the day, only reaching 70dB at the highest wind speeds. With the wind farm operating (yellow diamonds below), the range of results was generally similar. Note that the operational data is not all turbine noise; some periods will have peak sound levels caused by the same local ambient sounds captured in the pre-operational monitoring period.

(dBG weighting accentuates 10-30Hz, the threshold between audible low-frequency sound and infrasound, and includes 2Hz-70Hz)

At the closer home, a limited pre-operational monitoring period only captured wind from a couple of directions, so the report’s operational results only consider periods with these two wind directions, as well. (An appendix includes the full dataset of the operational period, which closely resembles that of the more distant home shown above, though peak sounds remain below 80dBG). In the limited dataset, pre-operational levels were significantly lower, clustered between 40-60dBG. After construction, the bulk of measurements were in the same range, though there was a clear increase in periods with measurements of 60-70db, with a few peaks up to 75dB. The authors of the report suggest that some of these higher measurements appear to be due to a transient non-turbine source (one chunk of them all occurred in one short period during which wind speed and direction did not change), and much of the rest may reflect higher daytime wind-related sound, rather than turbine sound, since the limited pre-operational period did not capture much data at high wind speeds. They also note that, regardless of the source, even these peaks were within the range recorded at the more distant site pre-operation, so they reflect sound “no greater than levels that occurred naturally in the local environment (prior to the) operation of the wind farm.”

A separate section of the report addresses audible low-frequency noise, using the dBA-lf metric (dBA weighting, applied only to sound from 10-160Hz), and also reported as linear (unweighted) results at each frequency band (down to 10Hz when compared to regulatory criteria, and to 0.8Hz in a series of charts of median levels in each frequency band). Again, results showed compliance with regulatory thresholds, except for a few 10-minute periods (roughly 2% of the periods); the authors of the report consider it likely that most of these are extraneous sounds, or would be in compliance if found to be steady, rather than variable, sounds.

(Ed. note: It must be mentioned that the authors of the report are exceptionally diligent in suggesting plausible alternatives to turbine noise for each of the occasions where operational sounds appear to be higher than pre-operational; on-site human monitoring would allow at least some of these ambiguous time periods to be more definitively characterized.)

This report offers some good, solid new data, collected over a relatively long period of time (10 days or so, rather than a single day) with a decent range of wind directions and with raw data collected down to below 1Hz. While affirming that infrasound remains well below the 95dBG human perceptual threshold and 85dBG regulatory threshold, and also generally below the frequency-band limits widely applied to low frequency noise (10-160Hz), a few limitations in the research design leave several key questions unexplored:

First, the houses used in the study were relatively far away from the wind farm. While there are some noise complaints at the distances studied (especially in Australia and New Zealand), the vast majority of neighbor complaints occur when turbines are closer, from a quarter to half mile especially, and out to three-quarters of a mile (a bit over 1km) with some regularity. This study takes the important step of recording inside sound levels, but with many complaints coming at half or quarter the distance of even the closer home here (and a tenth the distance of the further home), we are left without a clear idea of infrasound or low-frequency noise levels at such locations. This may be especially relevant to the low-frequency findings, since even at the greater distances, inside low frequency sound was much closer to regulatory limits than were infrasound levels.

Second, the primary data is presented as 10-minute average sound levels. In an attempt to consider whether they were missing important shorter-term variation, the researchers also looked at 1-minute averages, and for part of the data, 10-second averages. They found that the 10-second averages closely tracked the 10-minute averages, with a similar amount of variation. However, several acousticians have suggest that the negative effects reported by some neighbors are caused by much shorter pulses of low-frequency or infrasound: investigations have centered on the roughly once-per-second blade-pass frequency, and on even more rapid fluctuations that can only be captured when filtering sound at at time frames of 10 milliseconds, matching the sensitivity of human hearing. It’s very likely that the 1-second peaks would show higher peak levels than the 10-second averages and 10-minute averages; one such analysis found 1-second peaks of 5-8dB higher than 10-second averages, with variations of up to 30dBG or more around the average when measured at 10ms, leading to peaks 10-17dB higher than the ten-second average. While regulatory criteria rely on longer averaging times, human responses to much shorter-term peaks, and/or to short and long-term variability, may well underlie many of the more vehement complaints that occur even when turbines are meeting regulatory noise limits. Investigating this possibility more widely would help settle what is becoming a central question in community responses to wind farms.

Finally, even ten days of monitoring may well not capture conditions that are particularly troublesome for neighbors. No indication is offered as to whether the monitoring was scheduled with any consideration for “worst-case” noise conditions, especially times of high atmospheric turbulence, or seasons when complaints have been highest (operational monitoring took place in southern hemisphere summer and autumn). The report notes just one two-day period when the resident at one of the homes noted that the noise seemed particularly bothersome (results those nights were generally clustered within the typical scatter of data, though on the high side of the range).

While it may appear to some that these final points are nit-picking attempts to find any small reason to ignore the overall findings of this study, I offer them not so much as critique, but rather as a nudge to researchers, to dig deep enough to more definitively address some of the particular qualities of wind turbine noise that are being hypothesized as contributors to community responses to turbines. In particular, averaging times for noise analysis must be well below one second (eg 125ms, or one-eighth of a second) in order to capture the amplitude modulation that gives many turbines their distinctive pulsing or throbbing sound quality.

This study does a good job at assessing the wind farm’s infrasound and low-frequency sounds against the regulatory criteria; however, with community complaints being common even around projects in compliance, there’s a need for research that can help clarify whether wind turbine sound does—or does not—have unusual qualities or variability patterns that existing regulatory standards are not designed to address.

The 2013 field season of the 5-year Southern California Behavioral Response study is underway now. This research applies suction-cup tags to whales, which track the whales’ movements (dive patterns, speed, direction, etc.) while also recording the sounds the whales are hearing, including sounds of mid-frequency active sonar played underwater by the researchers under carefully controlled conditions. Earlier years’ results have begun to quantify the level of sound that can spur behavioral reactions in several species of whales, including the beaked whales that have appeared to be more sensitive to sonar sound, resulting in several stranding incidents over the past fifteen years. Most recently, two new papers reported that both blue whales and Cuvier’s beaked whales seem to avoid sonar sounds, and at times stop feeding, at sound levels below most current regulatory thresholds.

Results from studies off Southern California have quantified for the first time the reactions of Blue whales and Cuvier’s beaked whales to simulations of naval mid-frequency active sonar. In both cases, scientists found that whales tended to move away from sonar signals, and appeared to suspend feeding activity for an hour or more at times.

The Cuvier’s beaked whale results marked the first time this species had successfully been monitored during a controlled exposure to sound while wearing a temporary suction-cup “D-TAG” that allows researchers to track animal dive and movement patterns while also recording the sound level of the sonar signal that the animal is hearing. As with similar experiments done on other species of beaked whale, the two whales tagged in this study changed their normal dive patterns, paused or stopped echolocating for food, and waited longer at the surface after the sonar sound ended before they began diving normally again. The pause in foraging lasted for 6 hours in one whale, and at least 90 minutes for the other.

The whales’ behavior was changed at sound levels (89-127dB) that are far below the levels typically considered problematic by regulators (typically 160-180dB; though some Navy EIS’s use 120dB for beaked whales, because of their previously observed noise sensitivity). CORRECTION, 1/31/14: The current round of Navy EISs and NOAA permits consider exposures down to 120dB in their analysis of behavioral “takes” for all species.

Researchers concluded that “The observed responses included vigorous swimming and extended time without echolocation-based foraging, imposing a net energetic cost that (if repeated) could reduce individual fitness.” While they did not see rapid ascents from dives that would support an early theory that some beaked whales may suffer tissue damage similar to what human divers experience as “the bends,” they suggest that the disruption of normal dive and surface-resting patterns could affect the animals’ dive metabolism in ways we don’t yet understand. Also of interest in this study was an unexpected period during which a tagged animal was exposed to sound from a distant (over 100km) naval exercise; in that case, the animal showed no response, though received levels were similar (78-106dB); researchers suggest that the animals could tell that these signals were much more distant than the test signals, which were under 10km away.

The Blue whale results were a bit more ambiguous, as there was significant individual variation among the 12 whales that were tagged and exposed to sonar-like sounds. Some whales were foraging at the surface, some were deep-diving feeding, and some were diving but not feeding. Whales at the surface showed little response, while diving animals reacted more strongly, including some instances of clear avoidance (i.e., swimming away, or “horizontal displacement” in the research parlance).

While the Blue whale results were not as uniform as the Cuvier’s results, this is the first time that blue whales have been studied to see how they respond to mid-frequency sonar, and the researchers consider even the modest effects to be significant, especially since blue whale populations are not rebounding similarly to other large whales. As the researchers conclude: “our results suggest that frequent exposures to mid-frequency anthropogenic sounds may pose significant risks to the recovery rates of endangered blue whale populations, which unlike other baleen whale populations (i.e. humpback, grey and fin whales), have not shown signs of recovery off the western coast of North America in the last 20 years.”

Using a complex set of measurements of 54 behavioral metrics (including such factors as orientation angle to the sound, change in pitch or angle of descent or ascent, and the number of lunges per dive), and applying a statistical formula that resulted in the average “response” ratings on the left axis of the charts below, researchers found statistically significant changes three key areas. The chart below shows the clear, yet subtle, changes in dive patterns (a), body orientation (b), and horizontal displacement (c), especially among the deep-feeding animals:

Researchers note that the whale that showed the largest reaction stopped feeding as soon as hearing the sonar signal and swam away from the sound; it did not begin feeding again for an hour, during which time it would have eaten over a ton of krill, which is about the minimum amount a whale needs per day (i.e., it’s a metabolically significant loss).

The responses noted occurred at average peak received levels of 130-160dB, again, notably lower than most regulatory thresholds for behavioral responses, which range from 160-180dB. CORRECTION, 1/31/14: The current round of Navy EISs and NOAA permits consider exposures down to 120dB in their analysis of behavioral “takes” for all species; in fact, the bulk of behavioral responses for “low frequency cetaceans,” such as blue whales, is expected at exposures similar to those here. There was a large range of response ratings for both dive patterns and body orientation (the chart above shows the average among all individuals); the avoidance responses showed a more modest range of variability, except for the one extreme response noted above. Overall, the results confirm previously-observed importance of behavioral context: “Since some of the most pronounced responses occurred near the onset of exposure but other, higher level exposures provoked no response, the data suggest that the use of received level alone in predicting responses may be problematic and that a more complex dose – response function that considers behavioural contexts will be more appropriate. Management decisions regarding baleen whales and military sonar should consider the likely contexts of exposure and the foraging ecology of animals in predicting responses and planning operations in order to minimize adverse effects.”

The World Ocean Council, an “international, cross-sectoral alliance for private sector leadership and collaboration in Corporate Ocean Responsibility,” has launched a new initiative to address ocean noise issues. Planned to complement the ongoing efforts of the oil and gas industry’s Sound and Marine Life program and the International Maritime Organization’s ocean noise policy work addressing shipping noise, the WOO’s Marine Sound Working Group will be especially helpful in raising awareness of ocean noise issues among ocean industries—including ocean mining and port construction—that have been less involved in the issue over the past decade or so of intensive study.

In an interview after the initial meeting of the Marine Sound Working Group, co-chair Brandon Southall noted efforts to find alternatives or noise-masking techniques for some noisy activities in which the noise is a by-product, rather than a necessary component of the work; he also stressed ongoing efforts to better understand the widespread effects of chronic moderate noise, in contrast to researchers’ earlier focus on localized, acute effects of specific loud noise sources. See the full 6-minute interview with Brandon here.

A new study has found surprisingly high noise levels in a large iceberg tracked from the time it calved from the Antarctic ice sheet until it disintegrated and melted at sea. Three kinds of sounds dominated: early on, the iceberg scraped against the seafloor; later, it collided with another iceberg; and finally, it cracked into pieces and disintegrated within a couple of months. At times, the sounds were loud enough to be recorded thousands of miles away, near the equator, and during one especially loud day, the sound was equivalent to that of over 200 supertankers.

“You wouldn’t think that a drifting iceberg would create such a large amount of sound energy without colliding into something or scraping the seafloor,” said Robert Dziak, a marine geologist at OSU’s Hatfield Marine Science Center in Newport, Ore., and lead author on the study, who has monitored ocean sounds using hydrophones for nearly two decades. “But think of what happens why you pour a warm drink into a glass filled with ice. The ice shatters and the cracking sounds can be really dramatic. Now extrapolate that to a giant iceberg and you can begin to understand the magnitude of the sound energy.”

“The breakup of ice and the melting of icebergs are natural events, so obviously animals have adapted to this noise over time,” Dziak said. “If the atmosphere continues to warm and the breakup of ice is magnified, this might increase the noise budget in the polar areas. “We don’t know what impact this may have,” Dziak added, “but we are trying to establish what natural sound levels are in various parts of the world’s oceans to better understand the amount of anthropogenic noise that is being generated.”

Noise complaints around the Sheffield Wind facility in Vermont began soon after the turbines began turning; combined with complaints from other wind farms, the Vermont Department of Public Service initiated investigations. This week, a report was released summarizing the results from three days of noise monitoring outside the home of a family that has been especially affected by turbine noise. Unfortunately, the conditions on these days were not similar to those that cause the residents problems; and more generally, on none of the three days were investigators able to document the turbine sound levels (on one day there was virtually no wind and they weren’t operating; on the other two, wind was too strong to hear turbines, and not from a direction that brings turbine noise to the house).

Chris Recchia, commissioner for the Vermont Public Service Department, said that while the noise testing may help his department better understand how to evaluate wind noise in the future, he cannot draw conclusions from it. “The testing is not helpful in terms of determining wind noise,” he said. “It really is not particularly useful in making conclusions about the compliance of the turbines.”

“This was our first attempt at trying to do independent noise testing, but it brings up more issues than it probably answers,” he said. “One of them is having a standard inside someone’s house.”

The acousticians on site found daylong average noise levels of 30dBA on the relatively windless day, and 45dBA and 47dBA on the windy days; at no time were the turbines audible through the wind, leading the investigators to conclude that the turbines’ contributions to these levels were lower than the state limit of 45dBA. The inability to isolate turbine noise in their monitoring left them unable to predict the inside noise level, which by state regs should remain below 30dB; they had assumed they could capture outside turbine noise, so had not arranged to make recordings in the house itself. However, as the report says:

…it should be noted that in conversations with the Therriens, the three-day measurement period was not representative of the worst-case noise conditions that they experience. They are most impacted by the wind turbine noise when the winds are from the east and the south, and their residence is directly downwind of the wind turbines. If measurements are to be made that demonstrate these worst-case noise conditions, it may be necessary to greatly extend the time of measurement period to catch the particular operating and atmospheric conditions that cause the level of annoyance claimed by the Therriens.

UPDATE, 7/15/13: The Therriens and Vermonters for a Clean Environment have raised questions about the reported power output on one of the two windy nights. According to a letter filed with the PSB, conditions were actually similar to previous high-noise periods on one of the testing nights, but the power output charts in the noise monitoring report show surprisingly low power output during some high wind times in the wee hours of the 2nd day of testing. Luann Therrien noted, “Up now at 2am. Imagine our surprise we are not being rocked out of the house by turbine whoosh and jet sound. First time in a long time that we are hearing mostly normal wind sounds (during a time when the wind speed and direction were optimal for loud turbine noise).” It’s possible that the wind project was under some curtailment from local grid operators who didn’t need the power at that time; VCE wonders whether the wind farm operator knowingly feathered the blades to reduce sound during the testing, though a spokeman for First Wind said, “Of course, we don’t make any adjustments when testing is going on.” For more, see the VCE letter to the PSB.

Some larger issues are also spotlighted by this study. Charts included in the report offer a clear representation of the variability in wind noise over the course of the day—the daylong average levels, especially on the windy days, were far exceeded for much of the day. While in this case, the noise was wind, similar variability is commonly also found in wind turbine sound (often 5-10dB above a daylong average, and at times 15-20dB higher, though generally with lower peaks than this blustery day produced); daylong average figures, while useful in many ways, rarely reflect the actual noise experience of neighbors.

The difficulties encountered in this study highlight the need for noise monitoring—especially at homes with repeated complaints—to be planned with enough flexibility to be on site on days when weather forecasts predict the conditions that residents have stated to be the most troublesome, and to be sure turbines are operating at full power at times when conditions are ripe for issues. As noted in the comment section of the VtDigger piece (which features a lively, respectful discussion), those who are upset about turbine noise rarely say the turbines are always a problem; rather, there are often certain conditions that are significantly worse. To thus spend limited resources doing sound studies at randomly chosen times is likely to be of little practical use. The more troublesome conditions may occur more, or less, commonly in different locations, and may easily be missed by any brief monitoring period, unless spot monitoring is carefully and flexibly planned.

It should be stressed that even if the difficult conditions occur a relatively small proportion of hours per month, they can still create a chronic, hard to live with experience. For example, one study suggested peak turbine noise levels may occur as little as 4% of the hours in a year; but, doing the math on 4% of the time shows that this could mean 116 days—a third of the year—with peak sound for three hours a day, or 58 days—nearly two months worth of days—with peak sound for 6 hours; for more on this, and turbine sound variability in general, see this recent AEI presentation. (Note that this analysis is looking only at generalized yearly variability in wind-speed-driven turbine sound levels and some propagation factors, and does not incorporate any turbulence-induced increases in turbine sound levels; thus the 4% number is illustrative only, and not meant to represent actual rates of troublesome noise at any particular location. Few studies have looked at the effects of air turbulence or turbine wakes on turbine source levels, and none that I know of have actually tracked long-term patterns of sound variability around wind farms.)

RELATED, 7/23/13: More extended noise monitoring at another Vermont wind farm on Lowell Mountain has found no violations of the 45dB limit, including at times when turbines were operating at full capacity. Turbines were monitored continuously for two weeks in May and June, according to a local news report on the testing. Two earlier monitoring periods found a total of four hours in which turbines exceeded the noise limits; Green Mountain Power says this was due to snow build-up, and new equipment will allow them to shut down turbines if that happens again; a hearing in early August will determine whether Green Mountain Power will be penalized for the violations.

As one local chimed in, “I was there and found the sounds to be very emotive… I used to live at Marsden and the sound of the foghorn meant my father and other family members would be safe at sea. Thank you to ALL who helped create a magical, once-in-a-lifetime event.”